1. Field
Embodiments of this invention relate to the field of failure mechanisms found in chambers for inductively coupled plasma production, and more specifically, to a plasma chamber with an anodic coating with improved resistance to deterioration.
2. Description of Related Art
Inductively coupled plasmas (ICP) are routinely used in a number of different applications including production of activated gases, pollutant abatement and many others. In such devices, a coil is placed in close proximity to, around or within a vacuum chamber. When the coil is excited with radio frequency (RF), the electromagnetic fields induced around the coil sustain a gas plasma discharge within the vacuum apparatus. The plasma is coupled to the coil either through the air or through a magnetic core. In the latter case, the sources are called transformer coupled plasma (TCP) sources. The vacuum chamber is often a metal vessel that usually includes one dielectric gap to avoid the creation of a closed current loop through the chamber. As the metallic chamber is highly conductive, most of the induced voltage along the chamber drops across this gap.
Aluminum alloy is utilized in the fabrication of inductively coupled reactors for plasma production. During operation of the reactor, the interior of the plasma chamber is subjected to service in plasmas created from gases that are either corrosive, or in which dissociation results into corrosive products. One example of such gases is NF3, where its dissociation products include highly corrosive elements that attack the aluminum walls, such as atomic and ionized fluorine. To improve the resistance of the chamber to fluorine attack, an anodic coating is produced on the inner surface of the chamber.
In the course of plasma processing, surface damage occurs. In the particular case of fluorine attack, two specific types of fluorine attack have been observed: (i) a possible general attack in which the top of the anodized layer is enriched in fluorine (possibly AlFx); and (ii) a more localized attack in which nodules of presumably AlFx have formed. Furthermore, ion bombardment on the anodized metal housing results in pits that pierces the anodic layer exposing the underlying aluminum alloy substrate.
Factors that contribute to the deterioration of the anodic layer include: thermal stress, substrate imperfections, and ion bombardment, in conjunction with a chemically aggressive environment. Thermal stress relates to the plasma chamber being subjected to high temperatures and cycling. In some cases, peak temperatures in the plasma chamber can rise to the order of 140° C.–150° C. To reduce the temperature in the chamber, pipes can be placed in the plasma chamber, and cooling water run through the pipes.
The second category of factors that relate to damage of the anodic layer can be characterized as substrate-related. Two substrate-related problems appear to be responsible for the degraded performance of anodic coatings. One is the surface preparation prior to the anodization (a parameter controlled by the anodizer), and the other is the quality of the aluminum alloy utilized to manufacture the chamber.
Ion bombardment relates to the high-energy ions bombarding the coated surfaces of the chamber. As mentioned above, due to the chamber configuration and the way the power is coupled into the plasma, a voltage drop appears across the plasma sheath close to the chamber's dielectric break. That voltage drop accelerates the ions that impinge on the surfaces close to the gap.